Because of the steady depletion of easily obtainable carbon based fuels, such as petroleum and coal, there is an accelerating search for non-carbon based and renewable energy sources. One such renewable energy resource is wind power. Over centuries many types of devices designed to harness blowing wind to generate mechanical energy or, more recently, electrical energy have been developed. The famous windmills of the Netherlands and elsewhere have been and still are used for milling grain and pumping water out of low lying land areas.
Energy producing wind machines are usually called wind turbines and are broadly classified into two groups—horizontal axis wind turbines (HAWT) and vertical axis wind turbines (VAWT). Horizontal axis turbines are more prevalent and comprise a turbine that rotates around a horizontal axis. The main rotor shaft and generator are located at the top of a supporting tower and must be pointed into the wind by some means.
There are several common problems associated with HAWT machines. The more powerful horizontal axis turbines have long blades that require accurate placement out of the way of both natural and man-made obstructions and can create a safety hazard. They generate significant noise so as to cause some reluctance to have them installed near populated areas. In areas of high wind, the backward force and torque on a horizontal axis wind turbine blade peaks as it turns through the highest point of its arc. The tower hinders the air flow at its lowest point on the arc producing a local decrease in force and torque. These opposing high and low torque conditions can produce torsion on the bearings and support towers.
Vertical axis wind turbines overcome many of the problems of the HAWTs. The turbine of the VAWT spins on a vertical axis on top of the support tower thus making operation much safer for people on the ground near the tower. The VAWT is able to receive wind from any direction and therefore does not require any mechanical or computer-directed turning mechanism to keep the turbine facing in the right direction. The generator may be placed on or near the ground so the tower is not required to support the generator in addition to the turbine itself.
However, VAWTs do have drawbacks. Like HAWTs, vertical axis turbines usually have a pulsating torque that is produced during each revolution of the turbine. This occurs because of the lift and drag produced by the turbine as it spins. For each wind direction, there is a point in the revolution that produces the most lift and an opposing point that produces the most drag on the turbine. To reduce the pulsating effect, it is advantageous to achieve maximum turbine rotation as soon and efficiently as possible.
Coupled with the pulsating effect is the fact that two main types of VAWTs exist—the Savonius turbine which is a high speed, low torque turbine and the Darrieus turbine, which is a low speed, high torque turbine. Generally, the more efficient Darrieus turbine needs a starting motor or an additional Savonius rotor to start the Darrieus because the starting torque is low. The necessity of using additional starting devices reduces the efficiency of the Darrieus type of vertical wind turbine.
Thus, there exists in the field the problems of reducing the pulsating effect found in the rotation of vertical axis turbines and increasing the efficiency of vertical axis turbines.